The demand for so called dual mode terminals or mobile stations is expected to increase with time. In the United States of America there is presently at least one system which supports dual mode operation, i.e., digital time division multiple access (TDMA) and analog frequency modulation (FM), also referred to as DAMPS. As other systems such as GSM, PCS, DECT, DCS1800 and TDMA1900 become more widely used there is expected to be a need to have mobile stations which support two modes of operation, such as GSM and DCS1800 or DAMPS and TDMA 1900 or DCS1900. A proposed third generation mobile telecommunication systems, such as UMTS (in ETSI) or FPLMTS (in CCIR), is under development. These advanced systems may also require some type of dual mode operation.
In an article entitled "GSM and DECT-A Dual Mode Solution", Mobile Communications International Apr. 21, 1995, pgs. 57-60, B. Rashidzadel et al. describe an RF subsystem in FIG. 3 for a dual mode handset that operates in the GSM frequency band (890-960 MHz) and the DECT frequency band (1880-1900 MHz). In the transmitter portion a single I/Q modulator provides direct modulation at either the DECT or GSM frequency bands, and is connected through a SPDT switch to one of a DECT or a GSM transmitter chain. In the receiver portion separate DECT and GSM low noise amplifiers (LNAs) are used.
During the use of dual mode mobile stations an important consideration is power amplifier efficiency. Power amplifiers have a characteristic signal compression point. To improve efficiency it is typically desirable to operate as close to the compression point as possible. However, at the compression point the amplitude and signal phase changes (e.g., AM/AM and AM/PM conversion). This is not an important factor in the analog cellular mode, which has constant envelope FM modulation. However when using, for example, .pi./4 DQPSK modulation the operation near the compression point does become an important consideration.
Referring to FIG. 1, it can be seen that operation at the compression point results in the generation of higher order (e.g., third and fifth order) intermodulation products. The generation of the intermodulation products tends to spread the bandwidth of the transmitted signal such that a desired bandwidth (e.g., 30 KHz) is not achieved. This results in a leakage of transmitted signal energy into adjacent frequency channels, and thus an undesirable increase in interference between users.
It is known in the art to predistort the transmitted signal waveform in an attempt to compensate for the generation of the intermodulation products, and thus "linearize" the transmitted signal. For example, FIG. 2 shows a portion of a conventional approach to linearizing a transmitter signal. It is assumed that a mobile station includes a digital signal processor (DSP) section 1 and an RF section 2. The DSP 1 includes a modulator 1A that provides In-phase (I) and Quadrature (Q) signal channels to a predistortion block 1B wherein the I and Q digital values are modified (predistorted) to achieve a desired linearization of the transmitted waveform. The predistorted I and Q values are converted to analog signals with I and Q D/A converters 1C and 1D and applied to the RF section 2. In the RF section 2 the I/Q signals are mixed with a local oscillator (LO) signal (LO3) in a modulator 2A and upconverted to the transmission frequency. The modulated signal is applied to a gain controlled amplifier 2B, filtered by filter 2C, and power amplified by amplifier 2D before being applied at a transmission frequency (F.sub.TX) through a directional coupler 2E to a duplex filter 2F and antenna 2G. Signals received from antenna 2G at a receive frequency (F.sub.RX) are applied through the duplexer 2F to a low noise amplifier 2H and receive filter 21. The filtered received signal is next applied to a mixer 2J where it is mixed with a frequency supplied by LO1 and downconverted to a first intermediate frequency (IF), filtered by an IF filter 2K, and then applied to an IF signal processing block 2L where the IF is downconverted to baseband using a frequency locked to a voltage controller oscillator (VCTXO) 2M. The frequency of the VCTXO 2M is controlled by an automatic frequency control (AFC) signal output from a D/A converter 1L.
For predistortion linearization purposes it is necessary to generate a feedback signal from the transmitted signal. In FIG. 2 this is accomplished by providing a level-controlled amplifier A1 having an input coupled to an output of the directional coupler 2E. The output of A1 is applied to I/Q mixers 2N and 20 which are supplied by a separate mixing signal generated by a separate LO2. The desired phase shift between the LO2 inputs to the mixers 2N and 20 is provided by a LO phasing component 2P. The outputs of the mixers 2N and 20 are converted to digital values by A/D converters 1E and 1F. The digitized transmitter I/Q signals are supplied to the block 1B where an error calculation is performed in order to generate the required predistortion of the I/Q signals output from the modulator 1B, thereby maintaining the transmitted waveform within desired limits. The digitized I/Q signals from A/D converters 1E and 1F are also applied to a power calculation block 1G which provides an output to a summing node 1H where the calculated transmitter power is subtracted from a desired transmitter power level and ramp value generated by block 1I. The difference signal is applied to a loop filter 1J and then to a D/A converter 1K which supplies a gain control signal to the variable gain amplifier 2B, thus closing the power control loop.
It can be realized that the conventional approach to transmitter linearization outlined above and shown in FIG. 2 requires that additional components be provided, such as the amplifier A1, mixers 2N and 20, LO2, the A/D converters 1E and 1F, and the LO2 phasing network for the mixers 2N and 20. The addition of these components results in an increase in cost, size, complexity and power consumption, each of which is undesirable in the design and manufacturing of mobile stations, such as cellular radiotelephones.
While the transmitter linearization problem has been described above mainly in the context of TDMA-type terminals, similar problems arise in the operation of code division multiple access (CDMA)-type terminals.